To answer the question you asked, go to File / Properties and type in 1 or 2 for the frame rate. Note that this applies whether you're importing photos or not.

To answer the question i think you really meant to ask, which is "how do i make imported photos last for 1 or 2 frames?", go to Options / Preferences / Editing. Enter a shorter time for New still image length. 1 frame at 29.97fps would be about 0.033 seconds. 1 frame at 25fps would be 0.04 seconds.